Because of having a wide band gap, high electron saturation drift velocity, high critical electric field and high thermal conductivity, wide band gap compounds show great potential in application fields of high frequency, high temperature and high power. Especially, high-voltage nitride devices are drawing extensive attention of researchers all over the world because of their superior performance and great potential.
Conventional high-voltage nitride devices are normally manufactured by preparing a nitride epitaxial layer on a silicon substrate, and this manufacturing method is getting more and more mature. Epitaxial layer refers to that part grown and deposited on the substrate, and the nitride epitaxial layer is the nitride part of the epitaxial layer. Due to its low cost, this manufacturing method hugely promotes the marketing process of the high-voltage nitride devices.
When applying a high voltage to a high-voltage nitride device grown on a silicon substrate, the silicon substrate considered as a low resistance area is a path of the leakage current, since the critical electric field of silicon is low and the silicon material itself is electric conductive; and when the applied voltage is too high and the applied electric field exceeds the critical value, a breakdown may occur in the silicon substrate, which may further lead to a vertical breakdown of the nitride epitaxial layer. Generally speaking, the breakdown of the high-voltage nitride device grown on a silicon substrate is often caused by the vertical breakdown. When the silicon substrate is connected to ground, the breakdown voltage of the nitride devices on silicon substrate is reduced by half compared to that when the silicon substrate is floating. Although the breakdown voltage of the high-voltage nitride device with a silicon substrate mainly depends on the thickness of the nitride epitaxial layer, the nitride epitaxial layer on the silicon substrate is normally very thin, such as about 2 μm 7 μm. So the highest breakdown voltage of the high-voltage nitride device with a silicon substrate is normally less than 2000V, which is far less than the highest breakdown voltage of a high voltage nitride device grown on a SiC substrate or a sapphire substrate.
In order to increase the breakdown voltage of the high-voltage nitride device grown on a silicon substrate, the thickness of the nitride epitaxial layer and the voltage resistance of the silicon substrate may be increased. Due to the lattice and thermal mismatch between silicon and nitride layers, the thickness of the nitride epitaxial layer is greatly limited (about 2 μm 4 μm generally). In addition, a thicker epitaxial layer requires more material and more growth time, which greatly increases cost and decreases production capacity. Furthermore, the increasing of the thickness of the nitride epitaxial layer may lead to more defects including dislocations in the nitride epitaxial layer. Meantime, with the increasing of the working voltage, the leakage current can also be increased.
Another method to increase the breakdown voltage is removing the silicon substrate. However, the silicon substrate used for nitride layer growth is usually very thick (several hundred μm or even greater than 1 mm), and thus the etching process is complex and difficult. A new method to enhance the voltage resistance is required.
Since the applied voltage is generally loaded on the drain electrode of the device, the area between the gate electrode and the drain electrode is the main voltage withstanding area. Especially, when the silicon substrate is connected to ground, the area between the drain electrode and the silicon substrate electrode is the main voltage withstanding area, where the silicon substrate may breakdown more easily. To solve the above technical problems, a new high-voltage nitride device and a manufacturing method thereof are required.